The present invention relates to a magnetic recording medium suited for high-density recording.
Recently, in the magnetic recording media such as VTR tapes, audio tapes and computer tapes, there have been rising necessity of higher recording density to meet the requests for higher performance, longer recording time and miniaturization and light-weight of the recording media.
Shortening of the recording wavelength is essential for high-density recording, but when the thickness of the magnetic layer exceeds a certain level of thickness in relation to a particular recording wavelength, the magnetic flux of the magnetic substance in the depth of the layer is unable to pass through the reproducing head and forms a closed loop, resulting in a loss of the magnetic flux. Therefore, in principle, it is advantageous for magnetic recording that the more the recording wavelength is shortened, the more the magnetic layer is thinned. Based on this concept, an ME (metal evaporation) type magnetic recording medium has been developed in which the magnetic layer is formed with an extremely small thickness by depositing a ferromagnetic metal by a pertinent method such as evaporation or sputtering to realize the reduction of the magnetic flux loss by the thickness resulting from shortening of the recording wavelength.
However, this ME type magnetic recording medium is at a disadvantage in that the ferromagnetic metal film using cobalt, nickel or a mixture thereof used as magnetic substance tends to gather rust. Therefore, despite various proposals of improvement, this type of magnetic recording medium still lacks reliability for long-time preservation of recording.
On the other hand, in the case of the MP (metal powder) type magnetic recording medium in which a ferromagnetic metal is used as magnetic substance and this magnetic substance is dispersed in a resin binder and applied on a support film, it is relatively easy to take a measure against rusting, such as forming an anti-oxidizing film on the metal particle surfaces.
In the case of the coated type magnetic recording medium, however, there are involved the problems in coating, such as formation of pinholes or streaks, when the magnetic layer is reduced in thickness. It is also a negative factor against thickness reduction of the magnetic layer that it is hardly possible to increase the fill of the magnetic substance because of use of a resin binder containing various additives.
As a solution to these problems of the coated type magnetic recording medium, there has been proposed a magnetic recording medium in which the magnetic layer coated on the support film is divided into two layers so that a lower non-magnetic layer will be provided on the support film and an upper magnetic layer will be provided on the said non-magnetic layer. (See, for example, JP-A 4-270106 and JP-A 5-28464).
However, in the case where the magnetic layer is divided into two layers and the two layers are formed simultaneously by coating, disturbance tends to occur at the interface between the lower non-magnetic layer and the upper magnetic layer, resulting in generation of tape modulation noise. It has been proposed to first form the lower non-magnetic layer and then form the upper magnetic layer thereon while the lower non-magnetic layer is still in a wet state. This method, however, is still incapable of perfectly preventing disturbance at the interface, and further improvements have been desired.
As a result of the present inventors' earnest studies on the subject matter, it has been found that by forming a coating layer on the surface of a biaxially oriented polyester film composed of a polyester base film as a support film during the film forming process by an in-line coating method, the said coating layer being formed by an in-line coating method in the film forming process, and calendering the coated film to flatten its surface so as to satisfy the relations of the following formulae (1)-(4) in the said layers, the obtained magnetic recording medium has excellent adhesion between the polyester film and the coating layer, and is suited for use as a high-density recording medium in the shorter wavelength recording. EQU 0.01.ltoreq.tM.ltoreq.1.35 (1) EQU 0.05.ltoreq.t.ltoreq.1.39 (2) EQU 0.1&lt;t+tM.ltoreq.1.4 (3) EQU sRa&lt;10 (4)
(wherein t (.mu.m) is thickness of the coating layer, tM (.mu.m) is thickness of the magnetic layer and sRa (nm) is the average center line roughness of the surface of the said magnetic layer). The present invention has been attained on the basis of the above finding.